1. PHYS 1443 – Section 003 Lecture #13
Wednesday, Oct. 15, 2002
Dr. Jaehoon Yu
How are conservative forces and potential energy related?
Equilibrium of a system
General Energy Conservation
Gravitational Potential Energy and Escape Speed
Power
Homework #7 is due noon, next Wednesday, Oct. 22!
Remember the 2nd term exam (ch 6 – 12), Monday, Nov. 3!
Monday, Oct. 13, 2003
PHYS , Fall 2003
Dr. Jaehoon Yu

2. How are Conservative Forces Related to Potential Energy?
Work done by a force component on an object through a displacement Dx is
For an infinitesimal displacement Dx
Results in the conservative force-potential relationship
This relationship says that any conservative force acting on an object within a given system is the same as the negative derivative of the potential energy of the system with respect to position.
1. spring-ball system:
Does this statement make sense?
2. Earth-ball system:
The relationship works in both the conservative force cases we have learned!!!
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

3. Energy Diagram and the Equilibrium of a System
One can draw potential energy as a function of position  Energy Diagram
Let’s consider potential energy of a spring-ball system
What shape would this diagram be?
A Parabola Us
What does this energy diagram tell you?
Potential energy for this system is the same independent of the sign of the position.
The force is 0 when the slope of the potential energy curve is 0 at the position.
x=0 is one of the stable or equilibrium of this system where the potential energy is minimum.
Minimum Stable equilibrium
-xm xm x
Maximum unstable equilibrium
Position of a stable equilibrium corresponds to points where potential energy is at a minimum.
Position of an unstable equilibrium corresponds to points where potential energy is a maximum.
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

4. General Energy Conservation and Mass-Energy Equivalence
General Principle of Energy Conservation
The total energy of an isolated system is conserved as long as all forms of energy are taken into account.
Friction is a non-conservative force and causes mechanical energy to change to other forms of energy.
What about friction?
However, if you add the new form of energy altogether, the system as a whole did not lose any energy, as long as it is self-contained or isolated.
In the grand scale of the universe, no energy can be destroyed or created but just transformed or transferred from one place to another. Total energy of universe is constant.
Principle of Conservation of Mass
In any physical or chemical process, mass is neither created nor destroyed. Mass before a process is identical to the mass after the process.
Einstein’s Mass-Energy equality.
How many joules does your body correspond to?
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

5. The Gravitational Field
The gravitational force is a field force.
The force exists every where in the space.
If one were to place a test object of mass m at any point in the space in the existence of another object of mass M, the test object will fill the gravitational force, , exerted by M.
Therefore the gravitational field g is defined as
In other words, the gravitational field at a point in space is the gravitational force experienced by a test particle placed at the point divided by the mass of the test particle.
So how does the Earth’s gravitational field look like?
Where is the unit vector pointing outward from the center of the Earth E
Far away from the Earth’s surface
Close to the Earth’s surface
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

6. The Gravitational Potential Energy
What is the potential energy of an object at the height y from the surface of the Earth?
Do you think this would work in general cases?
No, it would not.
Why not?
Because this formula is only valid for the case where the gravitational force is constant, near the surface of the Earth and the generalized gravitational force is inversely proportional to the square of the distance.
OK. Then how would we generalize the potential energy in the gravitational field?
Because gravitational force is a central force, and a central force is a conservative force, the work done by the gravitational force is independent of the path. RE m ri Fg rf
The path can be considered as consisting of many tangential and radial motions. Tangential motions do not contribute to work!!!
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

7. More on The Gravitational Potential Energy
Since the gravitational force is a radial force, it performs work only when the path is radial direction. Therefore, the work performed by the gravitational force that depends on the position becomes
Potential energy is the negative change of work in the path
Since the Earth’s gravitational force is
So the potential energy function becomes
Since only the difference of potential energy matters, by taking the infinite distance as the initial point of the potential energy, we obtain
For any two particles?
The energy needed to take the particles infinitely apart.
For many particles?
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

8. Example of Gravitational Potential Energy
A particle of mass m is displaced through a small vertical distance Dy near the Earth’s surface. Show that in this situation the general expression for the change in gravitational potential energy is reduced to the DU=mgDy.
Taking the general expression of gravitational potential energy
The above formula becomes
Since the situation is close to the surface of the Earth
Therefore, DU becomes
Since on the surface of the Earth the gravitational field is
The potential energy becomes
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu

9. Escape Speed RE m h ME vi
vf=0 at h=rmax
Consider an object of mass m is projected vertically from the surface of the Earth with an initial speed vi and eventually comes to stop vf=0 at the distance rmax.
Because the total energy is conserved
Solving the above equation for vi, one obtains
Therefore if the initial speed vi is known, one can use this formula to compute the final height h of the object.
In order for the object to escape Earth’s gravitational field completely, the initial speed needs to be
This is called the escape speed. This formula is valid for any planet or large mass objects.
How does this depend on the mass of the escaping object?
Independent of the mass of the escaping object
Monday, Oct. 13, 2003
PHYS , Fall 2002
Dr. Jaehoon Yu